1. Field of the Invention
The present invention relates to laser ophthalmic surgery using a compact, low-cost, low-power laser system with a computer-controlled, non-contact process and corneal topography to perform corneal reshaping using either surface ablation or thermal coagulation.
2. Prior Art
Various lasers have been used for ophthalmic applications including the treatments of glaucoma, cataract and refractive surgery. For non-refractive treatments (glaucoma and cataract), suitable laser wavelengths are in the ranges of visible to near infrared. They include: Nd:YAG (1064 nm), doubled-YAG (532 nm), argon (488, 514 nm), krypton (568, 647 nm), semiconductor lasers (630-690 nm and 780-860 nm) and tunable dye lasers (577-630 nm). For refractive surgeries (or corneal reshaping), ultraviolet (UV) lasers (excimer at 193 nm and fifth-harmonic of Nd:YAG at 213 nm) have been used for large area surface corneal ablation in a process called photorefractive keratectomy (PRK). Corneal reshaping may also be performed by laser thermal coagulation currently conducted with Ho:YAG lasers using a fiber-coupled, contact-type process. However, the existing ophthalmic lasers as above described have one or more of the following limitations and disadvantages: high cost due to the high-power requirement in UV lasers for photorefractive keratectomy; large size and weight; high maintenance cost and gas cost (for excimer laser), and high fiber-cost for contact-type laser coagulation.
In light of the above, it is an object of the present invention to provide ophthalmic laser systems which offer the advantages of: low-cost, reduced size and weight, reliability, easy-operation and reduced maintenance. Another object of this invention is to provide a computer-controlled scanning device which enables use of a low-cost, low-energy laser for photorefractive keratectomy currently performed only by high-power UV lasers.
It is yet another object of the present invention to provide a refractive laser system which is compact, portable and insensitive to environmental conditions (such as vibration and temperature). This portable system may also be used for a mobile clinical center where the laser is transported by a van. It is yet another objective of the present invention to provide a non-contact process for corneal reshaping using laser thermal coagulation, where predetermined corneal correction patterns are conducted for both spherical and astigmatic changes of the corneal optical power.
The prior U.S. Pat. No. 4,784,135 to Blum, et al. and assigned to IBM teaches the first use of far ultraviolet irradiation of a biological layer to cause ablative photodecomposition. This patent teaches that using a laser beam housing a wavelength of 193 nm and an energy level of much greater than 10 mJ/cm.sup.2 /pulse can be used to photoablate corneal tissue without the build up of excess heat. The present invention on the other hand uses a process that allows the use of energy levels of less than 10 mJ/pulse in a process that still allows photoablation.
There are several prior art U.S. Patents relating to refractive surgery, or photorefractive keratectomy. A UV solid-state fifth-harmonic of Nd:YAG (or Nd:YLF) laser at 213 nm (or 210 nm), is disclosed in U.S. Pat. No. 5,144,630 by the inventor, J. T. Lin. U.S. Pat. No. 4,784,135 suggests the use of a UV laser with wavelengths less than 200 nm, in particular Argon Fluoride (ArF) laser at 193 nm, for non-thermal photoablation process in organic tissue. Devices for beam delivery and methods of corneal reshaping are disclosed in U.S. Pat. No. 4,838,266 using energy attenuator, and U.S. Pat. No. 5,019,074 using an erodible mask. Techniques for corneal reshaping by varying the size of the exposed region by iris or rotating disk are discussed in Marshall et al, "Photoablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomy" Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986). Tangential corneal surface ablation using ArF excimer laser or harmonics of Nd:YAG laser (at 532 and 266 nm) is disclosed in U.S. Pat. No. 5,102,409.
This prior art however requires high UV energy of (100-300 mJ) per pulse from the laser cavity or (30-40) mJ per pulse delivered onto the corneal surface, where large area corneal ablation using a beam spot size of about (4-6) mm which gives an energy density of (120-200) mJ/cm.sup.2. Moreover, the prior art Argon Fluoride excimer lasers operate at a repetition rate of (5-15) Hz and also limit the practical use of the tangential ablation concept which takes at least (5-10) minutes for a -5 diopter corneal correction in a 5-mm optical zone. The high energy requirement of the currently used Argon Fluoride excimer laser suffers the problems of: high-cost (in system, erodible mask and gas cost), high-maintenance cost, large size/weight and system are sensitive to environmental conditions (such as temperature and moisture).
The prior L'Esperance patent, U.S. Pat. No. 4,665,913, disclosed the method of a scanning laser for corneal reshaping. The proposed concept of this prior art, however, had never been demonstrated to be practical or to achieve the desired clinical requirement of smooth ablation of the corneal surface. This prior art is not practically useful and had not ever been demonstrated to be real because of the conditions in the art. A high-power laser of (100-200 mJ) is required in the prior art in order to obtain a useful beam with a substantially square spot size of 0.5.times.0.5 mm (see prior art, Col. 3, line 65 and Col. 4, lines 1-14) due to the low efficiency of obtaining such a beam, and which further requires a substantially uniform density (see Col. 13, line 30 and Col. 15, line 25). To achieve myopic correction, for example, the prior art (Col. 13, lines 61-66 and Col. 15 lines 60-65) proposes a smooth laser density increase with increasing scanning radius under the condition that a substantially uniform density of the scanning beam is required for a substantially uniform scan area (Col. 15, lines 20-28 of L'Esperance). Furthermore, L'Esperance teaches (Col. 4, lines 40-50) that a depth of 0.35 mm in an area of 6 mm diameter might be achieved in about 15 seconds when a beam spot of 0.5.times.0.5 mm is used and each pulse ablated 14 microns. The prior art proposes the method of having individual square beams (0.5.times.0.5 mm) scan to the fashion of exact matching of the square boundaries to cover the area of 6 mm, where the overlap among these individual beams should be avoided, otherwise excessive ablation near the boundaries of each 0.5.times.0.5 mm spot causes ridges. This is also part of the reason that the prior art requires a substantially square section of the individual beam with a substantially uniform density.
The L'Esperance U.S. Pat. No. 4,665,913 requires a complex apparatus to select a section of the beam which is substantially uniform in density within a substantially square spot "dot". The overall efficiency would be less than 10% from the output of the laser window to the corneal surface and requires, where a high power (at least 100 mJ) excimer laser than will be required than the Blum, et al. patent. It is almost impossible to match exactly the boundary of each square beam to achieve a substantially uniform scanned area even if each individual beam is perfectly uniform and square in shape and the smooth increase of the radius of scanned areas to obtain, for example, a myopic correction profile, would still be almost impossible to achieve for an overall smooth corneal surface. The successive sweep of the scan areas would always leave ridges between these sweeps. It should also be noticed that in L'Esperance's patent (Col. 18, lines 10-28) uses overlaps between each of the scanned areas to obtain the desired ablation profiles of myopic (or other) corrections. However, the ridges between each of the successive ablated areas are very difficult to avoid if within each scanned area the ablated profiles are not substantially uniform. In fact, one should expect a very rough surface on these ablated areas in addition to the regular ridges between each overlapped zones. One of the problems found in these teachings is that each required individual ablated area be substantially uniform and in a round or square shape, which is very difficult to achieve even if a perfectly uniform, square portion of a fundamental beam is produced using a complex apparatus for beam reshaping and having the high initial power.
It is not clear that L'Esperance has found a suitable scanning method or an effective method of selecting a perfect beam (with uniform density and well-defined shape) which would overcome the above-described difficulties and make the proposed teaching become practical in cost and design for any clinical uses. In fact, L'Esperance's scanning method has also been challenged by another prior art of Muller, U.S. Pat. No. 4,856,513, where the difficulties and problems of L'Esperance's teachings are discussed (see Col. 2, lines 1-40 of Muller's patent).
It is therefore a further object of the present invention to provide a method and apparatus for corneal reshaping by using software-driven new scanning patterns which do not require substantially uniform density or a specific spot shape. Contrary to L'Esperance's teachings, which suggest that there should be a perfect boundary match among each square beams and that excessive overlap should be avoided, the present invention proposes that a large portion (50%-80%) of overlap among the individual beams is necessary in order to achieve uniform ablated areas and a smooth profile without ridges. Furthermore, a low-power UV laser (0.1-2 mJ on corneal surface) at its bare-beam (having typically a 3-lop profile) without any beam reshaping is sufficient to achieve a smooth ablation surface based on the method proposed in the present invention, where computer-controlled beam overlap and orientation are employed. In addition to the surface quality problems, it is also impossible for L'Esperance to achieve any meaningful clinical results using his proposed techniques based on the present low-energy laser of (2-4) mJ from the output laser window and (0.1-2) mJ on corneal surface.
Therefore, another object of the present invention is to provide a new method of beam scanning which combines beam overlap and orientation for a random beam density distribution on the ablated corneal surface such that the individual beam profiles are not critical, where the focused beam (spot size of 0.1-1.2 mm) uses very low energy (0.1-2 mJ) and at its bare-profile is delivered onto the corneal surface in an averaged fashion. Uniform, near flat-top ablated areas of (1-9 mm in diameter) can be performed by the nonuniform starting-beam, but only when a set of specific predetermined overlap and orientation parameters are used. Portions of the theoretical background was published by the inventor, J. T. Lin, in SPIE Pro. vol 1644, Ophthalmic Technologies II (1991), p.p. 266-275.
One of the essential feature of the present invention for the photorefractive keratectomy process is to use a scanning device in a laser system which has high repetition rates, 50 to 50,000 Hz, but requires less energy, ranging between 0.05-10 mJ per pulse, or about 10 to 100 times less than that of the prior art. This new concept enables one to make the refractive lasers at a lower cost, smaller size and with less weight (by a factor of 5-10) than that of prior art lasers. Furthermore, these compact lasers of the present invention are portable and suitable for mobile clinical uses. To achieve beam uniformity and fast refractive surgery (30 to 60 seconds), a mathematical model of the beam overlap and ablation speed is also disclosed in the present invention.
For the laser thermo-keratoplasty (LTK) process, the prior art uses fiber-coupled contact-type procedure which involves the following drawbacks: (i) slow processing speed (typically a few minutes to perform eight-spot coagulation) which causes the non-uniform collagen shrinkage zone; (ii) circular coagulation zone which limits the procedure only for spherical type correction such as hyperopia; and (iii) the contact fiber-tip must be replaced in each procedure.
In the present invention, a computer-controlled scanning device is able to perform the laser thermokeratoplasty procedure under a non-contact mode and conduct the procedure many times faster than that of the prior contact-procedure and without cost for a fiber-tip replacement. Furthermore the coagulation patterns can be computer predetermined for specific applications in both spherical and astigmatic corrections. The flexible scanning patterns will also offer uniform and predictable collagen shrinkage.
For ophthalmic applications, it is another objective of the present invention to include but not limited to photorefractive keratectomy, laser thermokeratoplasty, epikeratoplasty, intrastroma photokeratectomy (IPK), phototherapeutic keratectomy (PTK), and laser-assisted keratomileusis (LAK).